Microelectronic industries, as well as other related industries are constantly reducing the feature size for constructing microscopic structures. Effective lithographic techniques are essential in this quest and require constantly improved radiation sensitive materials (resists). In order to resolve smaller structures, the wavelength of the exposing light has been reduced into the deep UV regions of 248 nm, 193 nm and 157 nm in addition to exposure to EUV or x-ray radiation. As the patterns and wavelengths become finer, the material properties of the resists used for pattern delineation have become more and more demanding. In particular, requirements of sensitivity, transparency, aesthetics of the image produced, and the selectivity of the resists to etch conditions for pattern transfer become more and more strenuous.
Advanced resists usually employ a technique called chemical amplification in which an acid generated by photolysis catalyzes a solubility switch from alkali insoluble to alkali soluble by removal of an acid sensitive (acid cleavable) group protecting an alkali-solubilizing moiety. The principle of chemical amplification as a basis for resist operation has been known for some years (see U.S. Pat. No. 4,491,628). Most chemically amplified resists have been designed around the use of acid sensitive carboxylic esters or acid sensitive hydroxystyrene derivatives.
However, chemically amplified resist systems have many shortcomings. One problem is standing wave effects, which occur when monochromatic light is reflected off the surface of a reflective substrate during exposure. These standing waves are variations of light intensity in the photoresist film dependent on photoresist film thickness caused by constructive and destructive interference of monochromatic light due to reflection. This in turn reduces resolution and causes line width variations. These line width changes are particularly troublesome for ever shrinking features. For example, standing waves in a positive photoresist have a tendency to result in footing at the photoresist/substrate interface reducing the resolution of the photoresist. CD variations can be controlled by tightly controlling the photoresist thickness variation. However, this is difficult to do when the photoresist has to cover steps in the substrate topography. Patterning over substrate topography can also cause localized reflections, which result in areas of the photoresist to be over or underexposed. This reflective notching results in localized line width variations.
In addition, chemically amplified resist profiles and resolution may change due to substrate poisoning. This effect occurs, particularly, when the substrate has a nitride layer. It is believed that residual N—H bonds in the nitride film deactivate the acid at the nitride/resist interface. For a positive resist, this results in an insoluble area, and either resist scumming, or a foot at the resist/substrate interface.
Furthermore, lithographic aspect ratios require the chemically amplified resist layer be thin, e.g., about 500 nm or lower, to print sub 180 nm features. This in turn requires the resist to have excellent plasma etch resistance such that resist image features can be transferred down into the underlying substrate. However, in order to decrease absorbance of the chemically amplified photoresists, aromatic groups, such as those in novolaks had to be removed, which in turn decreased the etch resistance.
The most common type of resists are called “single layer” resists (SLR). When employing light to expose, these resists utilize highly absorbing thin anti-reflective coatings (ARCs) to minimize standing wave effects. These anti-reflective coatings also mitigate problems with substrate poisoning if they are applied between the substrate and the photoresist.
In SLR systems a resist has the dual function of imaging and plasma etch resistance. Therefore, distinct performance tradeoffs have to be made between lithographic properties of the resist like absorbance, image profiles, resolution and substrate plasma etch resistance. For example in a typical dielectric mask open the resist is required to withstand an oxygen plasma etch to open up the ARC layer and a subsequent substrate etch. A significant amount of photoresist is lost during the ARC etch process as the etch selectivity between resist and ARC is only about 1:1. This requires the ARC to be as thin as possible to retain a sufficient photoresist film thickness for the substrate etch. Therefore, these ARCs must have relatively high absorbance (k-values) at the actinic wavelength in order to be effective in preventing substrate reflection.
Another approach is the utilization of a bilayer resist (BLR) system. A BLR system consists of a substrate coated with a first thick layer of Underlayer (UL) film followed by a second thin layer of dried resist (imaging layer or IL). The Underlayer film is etched in an oxygen rich plasma environment with the IL functioning as an etch mask. The typical imaging layer consists of a dried, chemically amplified, silicon containing resist. The silicon in the IL converts to silicon dioxide during the oxygen plasma etch of the UL, thus giving the IL the needed etch selectivity to the UL. The Underlayer film then acts as a etch resistant mask for substrate etching using non-oxygen or low-oxygen plasma etch chemistries, for removal of the underlying substrate.
Similar to the ARC in SLR, the Underlayer film in BLR should be optimally designed to absorb most of the deep UV light, which attenuates standing wave effects. In addition, the Underlayer film prevents deactivation of the acid catalyst at the resist/substrate interface. However, in the BLR system it is the Underlayer film that is the primarily etch mask for the substrate etch. Hence, Underlayer films incorporate polymers with functional groups to provide etch selectivity. The resist doesn't have the dual function to provide imaging and plasma etch resistance. Thus the resist can be thin compared to the SLR system and its lithographic properties do not have to be compromised. In addition, the Underlayer composition is applied to the substrate to produce an Underlayer film at about 2 to 5 times the thickness of a typical ARC layer. This helps substantially in planarizing the substrate prior to the next imaging step. Therefore the above mentioned reflectivity problems due to substrate topography as well as resist thickness uniformity problems have been improved.
Even though the underlayer film attenuates standing waves and substrate poisoning, it poses other problems. First, some underlayers are soluble to the chemical amplified resist solvent component. If there is intermixing between the resist when applied and the underlayer, the resolution and sensitivity of the imaging layer will be detrimentally affected.
The refractive index (N) is a complex number, N=n−ik, where n is the “real” part of the refractive index, and ik the imaginary part of the refractive index. k is related to the absorption coefficient as a function of wavelength, α(λ), by k=λα/(4π). If there is a large difference in the index of refraction between the chemically amplified resist and the underlayer film, light will reflect off the underlayer film which causes standing wave effects in the resist. Thus, the real portion “n” of the index of refraction of the two layers must be made to essentially match or to have their differences minimized, and the imaginary portion “k” of the index of refraction of the two layers must be optimized to minimize reflectivity effects. This optimization is important to minimize residual reflectivity from the UL/IL interface. Thus, when selecting the appropriate k-value a trade-off must be made between suppression of substrate reflectivity and reflectivity from the UL/IL interface. k-values in the UL can be low compared to ARCs because ULs are employed at a much higher film thickness. This results in low UL/IL interface reflectivity. ARCs on the other hand need to be thin because of poor etch selectivity to the photoresist and in order to control substrate reflectivity they need to be highly absorbing.
Another problem with underlayers is that they are sometimes too absorbent because of incorporation of aromatic groups. Some semiconductor manufacturing deep UV exposure tools utilize the same wavelength of light to both expose the photoresist and to align the exposure mask to the layer below the resist. If the underlayer is too absorbent, the reflected light needed for alignment is too attenuated to be useful. However, if the underlayer is not absorbent enough, standing waves may occur. A formulator must balance these competing objectives.
In addition, the underlayer film must be compatible with at least one edge bead remover acceptable to the semiconductor industry as well as give excellent compatibility with the resist coated over the underlayer film so that lithographic performance (e.g. photospeed, wall profiles, depth of focus, adhesion, etc.) are not adversely impacted.
Furthermore, some underlayers require UV exposure in order to form cross-links before the radiation sensitive resist can be applied to form the imaging layer. The problem with UV cross-linking underlayers is that they require long exposure times to form sufficient cross-links. The long exposure times severely constrain throughput and add to the cost of producing integrated circuits. The UV tools also do not provide uniform exposure so that some areas of the underlayer may be cross-linked more than other areas of the underlayer. In addition, UV cross-linking exposure tools are very expensive and are not included in most resist coating tools because of expense and space limitations.
Some underlayers are cross-linked by heating. However, the problem with some of these underlayers is that they require high curing temperatures and long curing times before the resist for the imaging layer can be applied. In order to be commercially useful, underlayers should be curable at temperatures below 250° C. and for a time less than 180 seconds. After curing, the underlayer film should have a high glass transition temperature to withstand subsequent high temperature processing and not intermix with the resist layer.
Even at temperatures below 250° C. sublimation of small amounts of the underlayer film components (e.g. TAGs, oligomers from the polymer, etc.) or products (e.g. acids, alcohols, water) from the thermal curing frequently occur. This can result in contamination of the equipment, which requires more frequent cleaning or replacement of equipment to avoid loss of product yield and other measures. This results in increased costs and is undesirable.
In certain applications, it is desirable for the underlayer film to planarize the surface of the substrate. However, this may be difficult to accomplish with underlayer films undergoing thermal crosslinking. As the temperature rises and the number of crosslinks increase, the glass transition temperature of the film increases. This makes it more difficult for the film to flow and planarize the substrate. Thus it is desirable to use as low a molecular weight material as possible to improve the planarization. However, use of lower molecular weight polymers in the underlayer film can adversely impact the lithographic performance of the resist imaging process.
In addition to the requirements for no intermixing, no sublimation, EBR compatibility, good planarization properties, and optical properties appropriately complementing the resist coated over the underlayer film, underlayer films need to have good etch resistance to allow the pattern transfer into the underlying substrate. While novolak or p-hydroxystyrene (PHS) based polymers show good etch resistance to various plasma sources, these materials absorb very strongly at 193 nm and their k(λ=193 nm) value is as high as 0.64. (Proc SPIE 2001, 4345, p 50 and EP Patent No. 0542008). Conversely, acrylate-based polymers possess good optical transparency at 193 nm but suffer from poor etch resistance. Solutions for these issues individually frequently adversely impact the performance in other areas. U.S. Pat. No. 6,610,808 (De et al.) describes a polymer for use in an underlayer film in a BLR system incorporating polar groups to improve EBR compatibility. US Patent application 2005/0238997 (De et al.) describes a polymer system with decreased sublimation tendencies and optimized MW ranges for the same use. However, these acrylic based underlayer film materials need improved plasma etch resistance for certain applications.
Cycloolefin polymers offer better transparency at 193 nm compared to novolaks and PHS-based materials. U.S. Pat. No. 6,136,499 discloses that cycloolefin polymer systems exhibit etch resistance superior to even some aromatic systems. However, it is not obvious how to employ these materials as an underlayer film and obtain all of the required properties of an underlayer film at the same time.